Matting agents are often used in photographic elements to provide a rough surface to the element, which is often desirable. Matting agents can provide an irregular surface to a photographic element, thereby permitting sufficient surface roughness to allow retouching or writing on the surface of the element. Surface roughness can also be desirable to prevent the surface of the photographic material from sticking to an adjacent surface and can provide a desired coefficient of friction to allow for use in apparatus for rapid handling and transport of the photographic material. Additionally, matting agents can help prevent the formation of Newton's rings when printing and enlarging because the area of contact of the surface of the photographic material with another material is relatively small due to the spacing effect of the matting agent. In lithographic photographic processes involving juxtaposing an unexposed photographic element with an original image that is desired to be copied, or an image-containing processed film element with a printing plate to impose an image on the plate, roughness on the surface of the film element imparted by a matting agent allows for relatively rapid vacuum draw-down between the film element and the original or plate.
Matting agents are usually present in a separate, overcoat layer of a photographic element, although they can be incorporated in a lower layer such as an emulsion layer as long as they impart roughness to the element. Examples of organic matting agents are particles, often in the form of beads, of polymers such as polymeric esters of acrylic and methacrylic acid, e.g., poly(methyl methacrylate), cellulose esters such as cellulose acetate propionate, cellulose ethers, ethyl cellulose, polyvinyl resins such as poly(vinyl acetate), styrene polymers and copolymers, and the like. Examples of inorganic matting agents are particles of glass, silicon dioxide, titanium dioxide, magnesium oxide, aluminum oxide, barium sulfate, calcium carbonate, and the like. Matting agents and the way they are used are further described in U.S. Pat. Nos. 3,411,907 and 3,754,924.
It is a common practice in the photographic art to coat more than one layer of a photographic element in a single pass through a coating machine. Such multilayer coating procedures are described, for example, in U.S. Pat. Nos. 2,761,791 and 3,508,947. These multilayer coating procedures often result in savings of time, effort, and expense in the coating of elements. In the coating of matting agent overcoat layers, such multilayer coating techniques (or at least coating the matte layer in a separate step while the underlying layer is still wet) are even more desirable, as they lead to improved adhesion of the matting agent to the photographic element, preventing washout of the matting agent during processing.
When such multiple wet layers are dried, drying proceeds from the surface inward, which tends to force the matting agent particles from the overcoat layer into the underlying emulsion layer. In many photographic elements, such as graphic arts photographic elements for use in preparing lithographic printing plates, rapid drying of the layers is desirable to improve the dimensional stability of the element. This rapid drying, however, aggrevates the problem of forcing matting agent particles into the emulsion layer of the element.
When such an element is imagewise exposed and processed, the image density in the area underlying a matting agent particle that has invaded the emulsion layer is diminished compared to other areas of the emulsion that have received equivalent exposure. These areas of decreased image density appear as small white spots in the image. The resulting visual effect has been called the "starry night" effect due to the similarity in appearance to a starry night sky.
Prior to the present invention, then, one was left with the choice of applying the matting agent to a dried layer, which leads to poor adhesion and wash-off of the matting agent during processing, or applying the matting agent to a wet layer, which leads to the starry night effect. It would therefore be highly desirable to provide a matting agent that can be incorporated in a photographic element in such a way that it does not wash off the element during processing and does not lead to the starry night effect, even if the element is subjected to rapid drying after coating.